EVP_DIGESTINIT(3)

📦 version 4.1.0, in opensuse-leap 16.0

💅 EVP_DIGESTINIT(3) — the EVP digest routines, explained by your resident crypto bestie

okay so this is a page from the Library Functions Manual, and it's all about the <openssl/evp.h> digest girlies — the high-level interface to message digests. yes, the whole squad is here: EVP_MD_CTX_new, EVP_MD_CTX_reset, EVP_MD_CTX_free, EVP_MD_CTX_init, EVP_MD_CTX_create, EVP_MD_CTX_cleanup, EVP_MD_CTX_destroy, EVP_DigestInit_ex, EVP_DigestUpdate, EVP_DigestFinal_ex, EVP_Digest, EVP_MD_CTX_copy_ex, EVP_DigestInit, EVP_DigestFinal, EVP_MD_CTX_copy, EVP_MD_CTX_md, EVP_md_null, EVP_sha224, EVP_sha256, EVP_sha384, EVP_sha512, EVP_sha512_224, EVP_sha512_256, EVP_ripemd160, EVP_get_digestbyname, EVP_get_digestbynid, and EVP_get_digestbyobj. all of them. no crumbs. ✨

🦋 SYNOPSIS 🦋

#include <openssl/evp.h>

EVP_MD_CTX *
EVP_MD_CTX_new(void);

int
EVP_MD_CTX_reset(EVP_MD_CTX *ctx);

void
EVP_MD_CTX_free(EVP_MD_CTX *ctx);

int
EVP_MD_CTX_init(EVP_MD_CTX *ctx);

EVP_MD_CTX *
EVP_MD_CTX_create(void);

int
EVP_MD_CTX_cleanup(EVP_MD_CTX *ctx);

void
EVP_MD_CTX_destroy(EVP_MD_CTX *ctx);

int
EVP_DigestInit_ex(EVP_MD_CTX *ctx, const EVP_MD *type, ENGINE *engine);

int
EVP_DigestUpdate(EVP_MD_CTX *ctx, const void *d, size_t cnt);

int
EVP_DigestFinal_ex(EVP_MD_CTX *ctx, unsigned char *md, unsigned int *s);

int
EVP_Digest(const void *d, size_t cnt, unsigned char *md,
    unsigned int *s, const EVP_MD *type, ENGINE *engine);

int
EVP_MD_CTX_copy_ex(EVP_MD_CTX *out, const EVP_MD_CTX *in);

int
EVP_DigestInit(EVP_MD_CTX *ctx, const EVP_MD *type);

int
EVP_DigestFinal(EVP_MD_CTX *ctx, unsigned char *md, unsigned int *s);

int
EVP_MD_CTX_copy(EVP_MD_CTX *out, EVP_MD_CTX *in);

const EVP_MD *
EVP_MD_CTX_md(const EVP_MD_CTX *ctx);

const EVP_MD *
EVP_md_null(void);

const EVP_MD *
EVP_sha224(void);

const EVP_MD *
EVP_sha256(void);

const EVP_MD *
EVP_sha384(void);

const EVP_MD *
EVP_sha512(void);

const EVP_MD *
EVP_sha512_224(void);

const EVP_MD *
EVP_sha512_256(void);

const EVP_MD *
EVP_ripemd160(void);

const EVP_MD *
EVP_get_digestbyname(const char *name);

const EVP_MD *
EVP_get_digestbynid(int type);

const EVP_MD *
EVP_get_digestbyobj(const ASN1_OBJECT *o);

🌸 DESCRIPTION 🌸

bestie. listen. the EVP digest routines are a high-level interface to message digests, and you should be using them instead of the cipher-specific functions. that's the whole vibe. 💅

  • EVP_MD_CTX_new() — allocates a new, empty digest context. fresh out the box, no notes. 🎀
  • EVP_MD_CTX_reset() — cleans up ctx and resets it to the state it had after EVP_MD_CTX_new(), so you can reuse it. recycling is main character energy. 🌈
  • EVP_MD_CTX_free() — cleans up ctx and frees the space allocated to it. say bye, release the memory, slay. 🦋
  • EVP_MD_CTX_init() — a deprecated function to clear a digest context on the stack before use. do not use it on a digest context returned from EVP_MD_CTX_new() or one that was already used. fr fr, don't. 💀
  • EVP_MD_CTX_create(), EVP_MD_CTX_cleanup(), and EVP_MD_CTX_destroy() — deprecated aliases for EVP_MD_CTX_new(), EVP_MD_CTX_reset(), and EVP_MD_CTX_free(), respectively. the old names. we don't use them anymore but we respect them. 🙈
  • EVP_DigestInit_ex() — sets up the digest context ctx to use a digest type. the type will typically be supplied by a function such as EVP_sha512(). the ENGINE *engine argument is always ignored and passing NULL is recommended. 👑
  • EVP_DigestUpdate() — hashes cnt bytes of data at d into the digest context ctx. you can call this several times on the same ctx to hash additional data. feed her, she's hungry. 🍓
  • EVP_DigestFinal_ex() — retrieves the digest value from ctx and places it in md. if the s parameter is not NULL, then the number of bytes of data written (i.e. the length of the digest) will be written to the integer at s; at most EVP_MAX_MD_SIZE bytes will be written. after calling EVP_DigestFinal_ex(), no additional calls to EVP_DigestUpdate() can be made, but EVP_DigestInit_ex() can be called to initialize a new digest operation. it's giving finale. ✨
⚠️ once you call EVP_DigestFinal_ex(), that's it — no more EVP_DigestUpdate() on that ctx. if you want to go again you gotta EVP_DigestInit_ex() first. that's the rule bestie.
  • EVP_Digest() — a simple wrapper function to hash cnt bytes of data at d using the digest type in a one-shot operation and place the digest value into md, and, unless s is NULL, the length of the digest in bytes into *s. this wrapper uses a temporary digest context and passes its arguments to EVP_DigestInit_ex(), EVP_DigestUpdate(), and EVP_DigestFinal_ex() internally. the ENGINE *engine argument is always ignored and passing NULL is recommended. one and done, iconic. 🔥
  • EVP_MD_CTX_copy_ex() — can be used to copy the message digest state from in to out. this is useful if large amounts of data are to be hashed which only differ in the last few bytes. literally copy-paste but make it cryptography. 🫶
  • EVP_DigestInit() — a deprecated function behaving like EVP_DigestInit_ex() except that it requires EVP_MD_CTX_reset() before it can be used on a context that was already used. 😭
  • EVP_DigestFinal() — a deprecated function behaving like EVP_DigestFinal_ex() except that the digest context ctx is automatically cleaned up after use by calling EVP_MD_CTX_reset() internally. she cleans up after herself, cute, but deprecated. 💅
  • EVP_MD_CTX_copy() — a deprecated function behaving like EVP_MD_CTX_copy_ex() except that it requires EVP_MD_CTX_reset() before a context that was already used can be passed as out. 🙈

🧚 the algorithm providers (aka the moodboard) 🧚

  • EVP_sha224(), EVP_sha256(), EVP_sha384(), EVP_sha512(), and EVP_ripemd160() — return EVP_MD structures for the SHA224, SHA256, SHA384, SHA512 and RIPEMD160 digest algorithms respectively. the classics. ⭐
  • EVP_sha512_224() and EVP_sha512_256() — return an EVP_MD structure that provides the truncated SHA512 variants SHA512/224 and SHA512/256, respectively. truncated and thriving. ✨
  • EVP_md_null() — a "null" message digest that does nothing: i.e. the hash it returns is of zero length. zero thoughts, head empty, no cap. 🫧

🔎 looking digests up 🔎

  • EVP_get_digestbyname(), EVP_get_digestbynid(), and EVP_get_digestbyobj() — return an EVP_MD structure when passed a digest name, a digest NID, or an ASN1_OBJECT structure respectively. three ways in, same destination. 🦋
  • EVP_get_digestbynid() and EVP_get_digestbyobj() are implemented as macros. iykyk. 😌

🎀 house rules 🎀

the EVP interface to message digests should almost always be used in preference to the low-level interfaces. why? because the code then becomes transparent to the digest used and much more flexible. it's giving glow-up. 🌈

the ENGINE *engine argument is always ignored and passing NULL is recommended. (yes, this is stated again. it's important. we're being thorough. periodt.) 💅

the functions EVP_DigestInit(), EVP_DigestFinal(), and EVP_MD_CTX_copy() are obsolete but are retained to maintain compatibility with existing code. new applications should use EVP_DigestInit_ex(), EVP_DigestFinal_ex(), and EVP_MD_CTX_copy_ex() because they can efficiently reuse a digest context instead of initializing and cleaning it up on each call. efficiency is hot. 🔥

💀 warning, bestie: if digest contexts are not cleaned up after use, memory leaks will occur. not cute. not slay. clean up your contexts.

🌟 RETURN VALUES 🌟

  • EVP_MD_CTX_new() and EVP_MD_CTX_create() return the new EVP_MD_CTX object or NULL for failure. 🎀
  • EVP_MD_CTX_reset(), EVP_MD_CTX_init(), and EVP_MD_CTX_cleanup() always return 1. always. no drama. 💖
  • EVP_DigestInit_ex(), EVP_DigestUpdate(), EVP_DigestFinal_ex(), EVP_Digest(), EVP_MD_CTX_copy_ex(), EVP_DigestInit(), EVP_DigestFinal(), and EVP_MD_CTX_copy() return 1 for success or 0 for failure. 🦋
  • EVP_MD_CTX_md() returns the EVP_MD object used by ctx, or NULL if ctx is NULL or does not have any message digest algorithm assigned yet. 👑
  • EVP_md_null(), EVP_sha224(), EVP_sha256(), EVP_sha384(), EVP_sha512(), EVP_sha512_224(), EVP_sha512_256(), and EVP_ripemd160() return pointers to constant static objects owned by the library. don't go freeing these, they're not yours. 🙈
  • EVP_get_digestbyname(), EVP_get_digestbynid(), and EVP_get_digestbyobj() return either an EVP_MD structure or NULL if an error occurs. 💅

🌈 EXAMPLES 🌈

okay so this example digests the data "Test Message\n" and "Hello World\n", using the digest name passed on the command line. look at her go:

#include <stdio.h>
#include <string.h>
#include <openssl/evp.h>

int
main(int argc, char *argv[])
{
        EVP_MD_CTX *mdctx;
        const EVP_MD *md;
        const char mess1[] = "Test Message\n";
        const char mess2[] = "Hello World\n";
        unsigned char md_value[EVP_MAX_MD_SIZE];
        unsigned int md_len, i;

        if (argc <= 1) {
                printf("Usage: mdtest digestname\n");
                exit(1);
        }

        md = EVP_get_digestbyname(argv[1]);
        if (md == NULL) {
                printf("Unknown message digest %s\n", argv[1]);
                exit(1);
        }

        mdctx = EVP_MD_CTX_new();
        EVP_DigestInit_ex(mdctx, md, NULL);
        EVP_DigestUpdate(mdctx, mess1, strlen(mess1));
        EVP_DigestUpdate(mdctx, mess2, strlen(mess2));
        EVP_DigestFinal_ex(mdctx, md_value, &md_len);
        EVP_MD_CTX_free(mdctx);

        printf("Digest is: ");
        for(i = 0; i < md_len; i++)
                printf("%02x", md_value[i]);
        printf("\n");

        return 0;
}

💜 SEE ALSO 💜

BIO_f_md(3), CMAC_Init(3), evp(3), EVP_BytesToKey(3), EVP_DigestSignInit(3), EVP_DigestVerifyInit(3), EVP_MD_CTX_ctrl(3), EVP_MD_nid(3), EVP_PKEY_CTX_set_signature_md(3), EVP_sha1(3), EVP_sha3_224(3), EVP_SignInit(3), EVP_sm3(3), EVP_VerifyInit(3), HMAC(3), OCSP_basic_sign(3), OCSP_request_sign(3), PKCS5_PBKDF2_HMAC(3), PKCS7_sign_add_signer(3), X509_ALGOR_set0(3), X509_digest(3), X509_sign(3)

⭐ HISTORY ⭐

a lil lore drop for you, bestie 🫶

  • EVP_DigestInit(), EVP_DigestUpdate(), and EVP_DigestFinal() first appeared in SSLeay 0.5.1.
  • EVP_md_null() and EVP_get_digestbyname() first appeared in SSLeay 0.8.0.
  • EVP_get_digestbynid() and EVP_get_digestbyobj() first appeared in SSLeay 0.8.1.
  • EVP_ripemd160() first appeared in SSLeay 0.9.0.
  • all these functions have been available since OpenBSD 2.4.

then:

  • EVP_MD_CTX_copy() first appeared in OpenSSL 0.9.2b and has been available since OpenBSD 2.6. 🦋
  • EVP_MD_CTX_md() first appeared in OpenSSL 0.9.5 and has been available since OpenBSD 2.7. 🎀
  • EVP_MD_CTX_init(), EVP_MD_CTX_create(), EVP_MD_CTX_cleanup(), EVP_MD_CTX_destroy(), EVP_DigestInit_ex(), EVP_DigestFinal_ex(), EVP_Digest(), and EVP_MD_CTX_copy_ex() first appeared in OpenSSL 0.9.7 and have been available since OpenBSD 3.2. ✨
  • EVP_sha224(), EVP_sha256(), EVP_sha384(), and EVP_sha512() first appeared in OpenSSL 0.9.7h and 0.9.8a and have been available since OpenBSD 4.0. 🔥
  • EVP_MD_CTX_new(), EVP_MD_CTX_reset(), and EVP_MD_CTX_free() first appeared in OpenSSL 1.1.0 and have been available since OpenBSD 6.3. 👑
  • EVP_sha512_224() and EVP_sha512_256() first appeared in OpenSSL 1.1.1 and have been available since OpenBSD 7.4. 🌸

and that's the page, bestie 🎀 this cutie came to you courtesy of the Debian folks, dated December 6, 2024, and it lives under the name EVP_DIGESTINIT(3). go hash something gorgeous 💖✨

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